Constant flow rate control valves have numerous applications in piping networks. For example, in a building air conditioning and heating system, water or other liquid at an appropriate control temperature is pumped from a central station through a piping network to various heat exchange units located throughout the building. Some of these heat exchange units are located relatively close to the central station while others are located much farther away. The fluid pressure applied across inputs and outputs of the respective heat exchange units varies widely because of factors such as frictional losses inherent in the flow of the liquid through the piping network and the distances the liquid must travel. To provide a constant flow rate in each branch of the network, constant flow rate control valves may be used.
A constant flow rate control valve provides a relatively constant fluid flow through the valve for varying pressure differentials. Therefore, constant flow rates within various branches of a piping network may be maintained even in the existence of varying fluid pressures within the branches. One example of a constant flow rate control valve is described in U.S. patent application 5,174,330, assigned to Flow Design, Inc. The '330 Patent describes a constant flow rate control valve that provides a constant flow rate for varying pressure differentials by regulating the amount of flow area through which fluid may flow. The regulation of the amount of flow area is performed by providing apertures in a portion of the device that are covered as the pressure differential increases. Such devices conventionally provide a relatively constant flow rate over a range of pressure differentials; however, they do not regulate flow for pressure differentials in excess of that range. Therefore, after a maximum pressure differential is exceeded, flow continues to flow unregulated and increases with increasing pressure differential.
A previously unrecognized problem with a valve continuing to provide increasing flow rate after a maximum pressure differential is reached, is that whenever the maximum pressure is exceeded, no flow is likely to be desired, rather than unregulated flow. For example, a constant rate control valve may be used to regulate flow of gasoline from a storage area to an output nozzle, such as used in gas stations. If for some reason, a conduit connecting the storage tank to the nozzle were to rupture or become flawed on the downstream side of such a control valve, the resulting loss in pressure in the downstream region would cause an increase in pressure differential across the valve. If such an increase exceeded the maximum pressure differential of a constant rate control valve, the rate of fluid flow through the valve and to the ruptured conduit would increase, and the gasoline would exit the ruptured conduit. Such flow of gasoline is dangerous. Therefore, in this example, after a maximum pressure is reached, it is desirable to prevent flow through the constant flow rate valve.